When Chrome came out, it was a new product; users had no expectations of "what Chrome is." So Google could do with it essentially whatever they wanted.
Firefox was in a completely different position. When Mozilla moved Firefox to rapid release, the product branded "Firefox" had been in users' hands for seven years -- nine if you count pre-1.0 versions. Over that time, users' expectations of what a product branded "Firefox" was settled into a particular place.
Rapid release was painful because it broke those expectations. Suddenly Firefox didn't behave like people expected a product labeled "Firefox" to behave anymore. It's like opening a bottle labeled "Coke" and having orange juice pour out. "But orange juice is better for you!" Yeah, but that's still not what I expect to get when I open a Coke bottle.
There's a simple way Mozilla could have avoided this: just call the rapid-release product something else. Create a new brand, and put it on a version of the browser that receives updates every six weeks. Call it Fastfox or Frequentfox or really anything other than Firefox. Then encourage users to start moving from Firefox to the new hotness. Make the new product compatible with Firefox extensions, but don't do the version-number compatibility check that Firefox does, so users aren't constantly being prompted to update working addons.
(Yes, both products are the same code under the hood. That doesn't matter. The important thing is that you communicate to users that this is a thing that is something other than Firefox, which resets their expectations.)
Eventually you'd have most users on the new product, since that's where the cool new features that get users excited would be showing up. Curmudgeons and enterprises would stay with boring old Firefox, but that's OK, because you make "Firefox" just a periodic snapshot of Frequentfox development. "Firefox" becomes a legacy brand, maintained for those who care about it. But the new brand is clearly established as the new hotness.
This lets you move your users without violating their expectations. They expect the new product to behave differently, because it's a new product. It's got a different name and everything!
Violent changes in direction for an established product, on the other hand, always tick off users, because they do violence to those users' expectations of what that product is.
It's better to send your well-loved legacy brand gracefully off into the sunset, in other words, than to try and shock new life into it with electric paddles.